Your next period is the first day of your last period plus your cycle length. Ovulation falls about 14 days before it, and the fertile window runs from 5 days before ovulation to the day after. This is an estimate, not medical advice, and not contraception.
The formula
Next period
next period = first day of last period + cycle lengthOvulation
ovulation = next period − luteal phase (14 days unless you change it)Fertile window
from ovulation − 5 days to ovulation + 1 dayEach later cycle starts on the day the one before it ends, so the same sums give three cycles. Ovulation is counted back from the next period rather than forward from the last because the second half of the cycle, from ovulation to the next period, is the steadier half. The NHS describes ovulation and fertility in the menstrual cycle, including how long sperm and the egg survive.
Worked example
With a 35-day cycle from 15 January, the next period is 19 February and ovulation is 5 February, on day 22: a longer cycle moves ovulation later by the same number of days.
When to use it, and the mistakes to avoid
Use it to plan ahead, to see roughly when to expect your period, or as a starting point when you are trying to conceive and want to know which days to watch.
- Treating it as contraception. It is not, and it cannot tell you that you are not fertile.
- Counting from the wrong day. The first day of bleeding is day 1.
- Using one unusual cycle. Enter your average length over several months.
- Expecting exact dates. Ovulation moves, so think in days either side.
- Ignoring symptoms. Irregular, painful or very heavy periods are worth a conversation with a doctor.
FAQ
How is the ovulation date worked out?
By counting back from the next period. Ovulation usually happens about 14 days before the next period starts, so the calculator adds your cycle length to the first day of your last period and takes 14 days off. With a 28-day cycle that is day 15. If you know your luteal phase (ovulation to the next period) is different, you can change it.
How is the fertile window worked out?
As the 5 days before ovulation up to the day after it. Sperm can survive in the body for several days, and the egg for about a day after it is released, so pregnancy is possible from sex in the days before ovulation as well as on the day. Calendar windows are a convention, and the real window shifts with your body.
How accurate is it?
Only as accurate as your cycles are regular. Ovulation can come earlier or later in a cycle because of stress, illness, travel, breastfeeding or coming off hormonal contraception, and the first half of the cycle varies most. Use it as a guide, and use ovulation test strips or a doctor's advice if timing matters.
Can I use it as contraception?
No. It is a calendar estimate and cannot tell you when you are ovulating, so you can get pregnant on days it does not mark as fertile. If you want to avoid pregnancy, use a reliable method of contraception and talk to a doctor or nurse about the options.
What if my cycle is irregular?
Then the dates will not be reliable, because the calculator assumes every cycle is the same length. Enter your average, treat the results as a rough guide, and see a doctor if your periods are very irregular, have stopped, or are unusually painful or heavy. They can help you work out what is going on.